During a well drilling operation, drilling fluids are circulated down the wellbore being drilled. The drilling fluid is generally pumped down the inside of the drillpipe and then passes through the drill bit into the wellbore. The fluid returns to the surface through the annulus, where it can then be recovered, processed, and reused. Drilling fluids perform a number of important duties during a drilling operation, such as lubricating and cooling the drill bit and removing generated rock cuttings. Maintaining sufficiently high viscosities of drilling fluids to provide effective suspension and removal of cuttings, and to provide effective fluid loss control, can be challenging, especially under high temperature conditions that can be experienced downhole.